Parathyroid hormone-related protein (PTHrP) is recognized as an important developmental regulatory molecule. In the mature mammal PTHrP is produced in vascular and other smooth muscle in association with normal physiological stimuli. Our hypothesis is that PTHrP and the PTH/PTHrP type I receptor are components of a novel compensatory autocrine vasoactive loop in which PTHrP acts to oppose pressor, proliferative and migratory cues. Recently we began to test this hypothesis in vivo by targeting PTHrP and its receptor to smooth muscle of transgenic mice. These "gain of function" models exhibited a reduction in basal blood pressure and vascular properties consistent with our hypothesis that PTHrP functions as a local vasorelaxant. Remarkably, over-expression of PTHrP was associated with striking abnormalities in heart development. In this proposal, we will further define the biology of PTHrP by analyzing the effect of removal of PTHrP and its receptor from vascular smooth muscle and heart using Cre-mediated gene deletion techniques. Aim 1 defines the requirement of PTHrP and the PTHrP-R for normal maintenance of cardiovascular and reno-vascular hemodynamics and vascular tone. Mouse models that are deficient or devoid of PTHrP or PTHrP-R in smooth muscle will be developed using Cre-mediated gene replacement strategies. Total body and renovascular hemodynamic measurements will be analyzed in the basal state and in response to hypertensive stimuli. Studies will be performed in isolated aortic and portal vein preparations to determine probe specific mechanisms by which PTHrP alters vascular tone. Aim 2 determines the importance of PTHrP and its receptor on proliferation and migration of vascular smooth muscle cells following arterial injury. We will examine the effects of manipulations in the expression of PTHrP activity in injury that induces a reproducible measure of VSMC hyperplasia. Aim 3 determines whether PTHrP is required during early heart development. The temporal and spatial expression of PTHrP and the PTHrP-R in the developing cardio- vascular system will be determined using in situ hybridization and immunohistochemistry. Ultrastructural studies and cardiomyocyte proliferative rates (BrdU) will be assessed in PTHrP: PTHrP over- expressing and the global PTHrP-R knockout embryos. We will determine the consequence of cardiac-specific ablation of PTHrP and its receptor on heart development by crossing a beta-myosin heavy chain driven Cre mouse with the PTHrP and PTHrP and PTHrP-R floxed mice.